Finding Alien Earths

Today's selection-- from Alien Earths by Dr. Lisa Kaltenegger. Discovering planets in other parts of the galaxy is hard:

“The discovery of new worlds outside our solar system started with a mystery: a tiny wobble. In 1995 two Swiss astronomers, Michel Mayor and Didier Queloz, whom we met earlier, detected a weird signal from the star 51 Pegasi. The star, a near twin to our own Sun, about fifty light-years away from Earth, unexpectedly wobbled back and forth on its stellar journey. And stars don't wobble for no reason. 

“Majestic Jupiter, the biggest planet in our solar system, contains the vast majority of the material left over from our Sun's creation and provided the first clues as to what afflicted 51 Pegasi. Jupiter makes our Sun wobble just a tiny bit. Jupiter is a humongous ball of swirling gas around a rocky core a dozen Earths big. This colossal gas giant, the fifth planet from the Sun, out beyond Mars, is a sight to behold: stunning patterns of storms cover the whole planet. Monstrous weather systems stir and twist the gases and paint the planet in patterns that look like van Gogh's The Starry Night. 

“If Jupiter were an empty box, all the other planets could fit into it and there'd still be room to spare. Jupiter dwarfs the Earth; you would need to put seventy Earths, one next to the other, to make a belt for Jupiter's middle (a fresh idea for a Halloween costume!). Powerful wind speeds exceeding 400 miles ( ~ 600 km) per hour create some of the largest storms in the solar system. One of them, Jupiter's Great Red Spot, has been observed for over a century—and is large enough to easily engulf Earth. Voyager 1—the spacecraft carrying the Golden Record out of the solar system-sent back the first detailed images of this gigantic storm in 1979. 

“But compared to the Sun, Jupiter is a lightweight. If Jupiter were a tablespoon of water, the Sun would be a four-gallon jug. If you had a cosmic set of scales, you would need about a thousand tablespoons of water on one side (a pile of Jupiters) to balance the Sun on the other side. In this comparison, Earth would be the size of a waterdrop. To balance the Sun on these cosmic scales, you'd need to place about three hundred thousand waterdrops (a humongous amount of Earths) on the other side. All the planets in our solar system together would make the cosmic scale tilt only a minuscule bit. The Sun is just so massive. The disk surrounding a nascent star contains only a tiny part of the material that creates the star at its center, and that disk forms all of its planets. 

“It would take about a hundred Earths to span the diameter of the Sun. To imagine this, line up one hundred peppercorns on the floor. (Pro tip: it is extremely helpful if the peppercorns are not the same color as the floor. In my first trial, I used black peppercorns on a dark floor, which, in hindsight, was not the smartest choice.) The one-hundred-peppercorn line shows the vast size of the Sun compared to our Pale Blue Dot. You would need more than one million Earths to fill the inside of the Sun ( volume is proportional to radius cubed). 

Sunset studies on Titan by Cassini help understand exoplanet atmospheres (artist's concept).

“So finding an exoplanet in the vast cosmos is extremely hard. If you wanted to find a planet circling another star, what kind would be the easiest to locate? Astronomers looked around our solar system and chose as their prototype the biggest, most massive planet to look for somewhere else: Jupiter. 

“The Sun's gravity loses some of its pull at the colossal giant planet's distance, so Jupiter does not need to travel as fast as the Earth to counter its gravitational pull. The balance between gravity and speed determines how long it takes for a planet to complete a circle around its star. While Earth does it in one year, Jupiter takes a leisurely eleven Earth years to circle the Sun. Knowing they would have a slightly easier time finding massive planets like Jupiter than finding tiny Earths, astronomers settled in for a decade-long search. 

Like the other giant planets in our solar system, Jupiter consists mostly of gas and ice because it formed far enough away from the hot Sun that ice and gas did not evaporate, leaving massive amounts of planet-building material as we have seen. It's cold beyond the ice line. Jupiter receives only one photon for every twenty-five photons Earth gets. 

“Planets are different than stars in more ways than just their size. They do not have nuclear-fusion reactors in their cores, so they do not produce energy and they do not shine. Like our Moon, they just reflect the starlight that hits them. That makes planets tiny, dim objects, incredibly hard to spot beside a huge, illuminating star. Seen from space, the Sun is more than one billion times brighter than Earth to our eyes. Think of it this way: one billion seconds is about thirty-one and a half years. If we compare the numbers not in brightness but in time, you would have to wait more than thirty-one and a half years of starlight to get one second of light from the planet. The light of an Earth is drowned out by the light of its host star. 

“But planet hunters have clever ways of finding their quarry. When you look up at night, you can see thousands of stars moving across the sky. In most cases, what appears to be the motion of the stars is actually the result of the Earth rotating around its axis and circling the Sun. But sometimes, there is an additional, unexpected motion, an indication that we’ve spotted something truly spectacular. Because even lightweight planets tug—just a little—on their heavyweight hosts. Both the star and its planet counter the other's gravitational pull by adding a little extra to their movements. Because the star is so much more massive, it wobbles only slightly if a planetary companion tugs on it. But that tiny wobble makes all the difference. It led astronomers to discover the first new worlds on our cosmic shore.”

Alien Earths: The New Science of Planet Hunting in the Cosmos
author: Dr. Lisa Kaltenegger  
title: Alien Earths: The New Science of Planet Hunting in the Cosmos  
publisher: St. Martin's Press  
page(s): 159-163